This paper summarizes the findings of investigations made during the past few years on serotonin. Three classes of naturally occurring alkaloids, the ergot alkaloids (LSD, Gynergen and ergotoxine), the harmala alkaloids, and yohimbine, have been shown to have in common: an indole nucleus and a substituted amino-ethyl side-chain; competitive antagonism of serotonin; and, at least one member of each class causes mental aberrations, e.g. LSD. It is thought that the mental changes induced by such drugs may be the result of interference with the action of serotonin in the brain. However, if this were the case, serotonin should reverse the mental effects of the drugs. Studies in mice with LSD and medmain showed that serotonin failed to overcome the neurological effects of the drugs.